console.log(Array.prototype);
//没有求和的方法
Array.prototype.sum = function() {
  var sum = 0;
  for (let i = 0; i < this.length; i++) {
    sum += this[i];
  }
  return sum
}
let arr = [1, 2, 3, 4]

console.log(arr.sum());

console.log(Array.prototype);